Machine tool probe

ABSTRACT

A probe for position determining apparatus such as a machine tool transmits optical measurement signals to a receiver module. The probe is battery powered and the battery powers light emitting diodes which transmit the optical measurement signals. The optical measurement signals comprise serially transmitted codewords which are generated by a universal synchronous/asynchronous receiver transmitter (USART). The output of the USART is combined with a pulse width modulator which has an output having the same form as the output of the USART but with reduced duration of each pulse, thus enhancing the life of the battery powering the light emitting diodes. The pulse width modulator may also be used to set the timing of the USART.

This invention relates to probes for use on position determiningapparatus such as coordinate measuring machines, measuring robots and inparticular machine tools.

An example of such a probe is shown in U.S. Pat. No. 4,153,998. Probesintended for use on machine tools in which there is a wireless signaltransmission between the probes and the controller of the machine toolare shown in European Patent Nos. 337669 and 337670.

When used on machine tools such probes are commonly battery-operated.These probes may transmit measurement signals optically to a receivermodule however, the power consumption in sending these optical messagesreduces the battery life.

The present invention provides a probe for position determiningapparatus comprising:

-   -   signal generating means for generating a signal;    -   signal transmitting means for transmitting the signal generated        by the signal generating means to a receiver module in the form        of optical pulses;    -   characterised in that a pulse width modulator is provided to        reduce the duration of each pulse.

Preferably the signal generating means includes a universalsynchronous/asynchronous receiver transmitter. The pulse width modulatormay control the timing of the universal synchronous/asynchronousreceiver transmitter (USART).

Preferably the probe is battery powered. The optical pulses may begenerated by LEDs.

A preferred embodiment of the invention will now be described withreference to the accompanying drawings wherein:

FIG. 1 is a diagrammatic view of the probe on a machine tool;

FIG. 2 a is part of a codeword of optical pulses formed by the universalsynchronous/asynchronous receiver transmitter (USART);

FIG. 2 b is the output of a pulse width modulator;

FIG. 2 c is the output of the universal synchronous/asynchronousreceiver transmitter (USART) and pulse width modulator combined; and

FIG. 3 is a schematic diagram showing the formation of a serialcodeword.

Referring to FIG. 1, the probe 10 is mounted in the spindle 12 of amachine tool exchangeably with the normal cutting tools. The spindle 12can move the probe in three dimensions x, y, z relative to a workpiece14 clamped on a table or bed 22 of the machine tool. Measurements aremade by contact between the stylus 11 of the probe and the workpiece.Measurement signals from the probe are transmitted optically asindicated by arrow 16 to a receiver module 19 mounted on a fixedstructure 20 of the machine tool. The probe 10 is battery operated. Thebattery powers LEDs located within window 18 which are used to transmitthe optical measurement signals.

The optical measurement signals indicated by arrow 16 in FIG. 1 compriseserially transmitted codewords. Each codeword is 8 bits long andcomprises a sequence of on/off pulses. These codewords are generated bysignal generating means, comprising a universal synchronous/asynchronousreceiver transmitter (USART). A section of codeword formed in this wayis shown in FIG. 2 a. Typically each pulse is 8 microseconds long and anew codeword may be transmitted every 16 milliseconds.

Using the universal synchronous/asynchronous receiver transmitter(USART) alone to generate the codewords has a disadvantage that as eachpulse is relatively long this results in high power consumption and thusreduced battery life.

To overcome this disadvantage the universal synchronous/asynchronousreceiver transmitter (USART) is combined with a pulse width modulator.As shown in FIG. 2B, the pulse width modulator generates a regularseries of pulses of shorter pulse length and with a shorter timeinterval between pulses than generated by the universalsynchronous/asynchronous receiver transmitter (USART). The time betweenpulses is typically 8 microseconds and pulse length is typically 2microseconds. The universal synchronous/asynchronous receivertransmitter (USART) output is combined with the pulse width modulatoroutput to provide a chopped output for transmission. As shown in FIG. 2c the form of this resultant output is the same as the form of theoutput of the universal synchronous/asynchronous receiver transmitter(USART) shown in FIG. 2 a, however the duration of each ON pulse hasbeen reduced to equal that of the pulse width modulator. This reducesthe duration of each LED flash and therefore reduces the powerconsumption of the system. Battery life is therefore enhanced.

Use of the pulse width modulator in combination with the universalsynchronous/asynchronous receiver transmitter (USART) has a secondadvantage. In previous systems without the pulse width modulator themicroprocessor must control both the timing and the sequence of thepulses, however when a pulse width modulator is incorporated into thesystem this may be used to set the timing of the universalsynchronous/asynchronous receiver transmitter (USART), i.e. it acts as aserial clock for timing. The microprocessor is used less as it is onlyrequired to send on/off messages to the universalsynchronous/asynchronous receiver transmitter (USART) and therefore hasimproved multi-tasking.

The pulse width modulator may already be incorporated into the probe forother uses such as motor control or digital/analogue conversion. The useof existing components in the probe for the system therefore reducescomponent cost.

FIG. 3 is a schematic diagram showing the formation of the serialcodewords. A microprocessor 30 in the probe incorporates a pulse widthmodulator 32 and signal generating means in the form of a universalsynchronous/asynchronous receiver transmitter (USART) 34. An output 36from the pulse width modulator 32 controls the timing of the universalsynchronous/asynchronous receiver transmitter (USART). Outputs 38,40 ofthe pulse width modulator 32 and the universal synchronous/asynchronousreceiver transmitter (USART) 34 are combined at an AND gate 42 to form aserial codeword 44. This serial codeword is transmitted from an LED orother signal transmitting means to an external receiver.

1. A probe for position determining apparatus comprising: signalgenerating means for generating a signal; signal transmitting means fortransmitting the signal generated by the signal generating means to areceiver module in the form of optical pulses; characterised in that apulse width modulator is provided to reduce the duration of each pulse.2. A probe according to claim 1 wherein the signal generating meansincludes a universal synchronous/asynchronous receiver transmitter.
 3. Aprobe according to claim 2 wherein the pulse width modulator controlsthe timing of the universal synchronous/asynchronous receivertransmitter.
 4. A probe according to claim 1 wherein the probe isbattery powered.
 5. A probe according to claim 1 wherein the opticalpulses are generated by light emitting diodes.
 6. A probe according toclaim 2 wherein the probe is battery powered.
 7. A probe according toclaim 3 wherein the probe is battery powered.
 8. A probe according toclaim 2 wherein the optical pulses are generated by light emittingdiodes.
 9. A probe according to claim 3 wherein the optical pulses aregenerated by light emitting diodes.
 10. A probe according to claim 4wherein the optical pulses are generated by light emitting diodes.